This invention relates to a voltage controlled oscillator (VCO) for generating a signal with a frequency which is varied by changing an input voltage.
In a PLL (phase locked loop) frequency synthesizer, for example, it is necessary to always keep the frequency of its output signal equal to a set frequency. To realize this a VCO, for example, is used.
One of the known VCOs is disclosed and illustrated in FIGS. 1 and 3 of Japanese Kokai No. 59-62215.
The illustration of FIG. 1 shows a circuit diagram of the VCO of the Kokai. A description of the VCO follows. A current dependent on an input voltage Vin supplied to an input terminal 1 flows through an input N channel MOS transistor 2. This current also flows through a P channel MOS transistor 3. The transistor 3, together with another P channel MOS transistor 4, constitutes a current mirror circuit. The transistor 4 serves as a current source for generating a current I4 corresponding to the input voltage Vin. A capacitor 9 is bidirectionally charged through a current path including a P channel MOS transistor 5, the capacitor 9, and an N channel MOS transistor 8, which provides the current flow indicated by the dotted line, or another current path including a P channel MOS transistor 7 and an N channel MOS transistor 6, which provides the current flow indicated by the solid line. The voltage at the terminals of the capacitor 9 is applied to voltage comparators 10 and 11. The comparators 10 and 11 may, for example, be inverters or differential circuits. The output signal from the comparator 10 is applied as a set input to a flip-flop 12. The output signal of the comparator 11 is applied as a reset signal to the same flip-flop 12. The Q output signal of the flip-flop 12 controls the turning on and off of the transistors 5 and 6. The Q output signal of the flip-flop 12 controls the turning on and off of the transistors 7 and 8.
In the FIG. 1 circuit, when the Q output signal of the flip-flop 12 is logical "1", the transistors 6 and 7 are turned on. Under this condition, a current flows along the path of the solid line to charge the capacitor 9. When the potential at one end of the capacitor 9 reaches the threshold voltage level of the comparator 11, the logical state of the flip-flop 12 is inverted. Then, the Q output of the flip-flop 12 becomes "0", and the Q output becomes "1". With these logical levels, the transistors 5 and 8 are turned on. As a result, the current path indicated by the broken line is formed to charge the capacitor 9 with the current I4. When the potential at the other end of the capacitor 9 is increased to reach the threshold voltage level of the comparator 10, the logical state of the flip-flop 12 is again inverted. The logical states of the Q output and the Q output are returned to the original state.
As described above, the capacitor 9 is bidirectionally and alternately charged. The charged voltages across the capacitor 9 are compared by the comparators 10 and 11, respectively. The flip-flop 12 is controlled by each of the output signals from the comparators 10 and 11. With a repetition of the sequence of the operation steps, an output terminal 14 of an inverter 13 coupled for reception with the Q output of the flip-flop 12 provides an output signal with a frequency corresponding to the input voltage Vin.
As shown in FIG. 3 of the above Kokai, it is proposed that, in the interest of improving the operating accuracy of the circuit, a voltage controlled oscillator including a couple of capacitors be used in place of the single capacitor 9 used in the first prior example.
In the voltage controlled oscillator shown in FIG. 1, at least two types of circuits, an inverter and a differential circuit, may be used for each comparator 10 and 11. When using the inverters, the theshold values of the transistors, when manufactured, are inevitably not invariable. This results in nonuniform threshold voltage values in the manufactured inverters, an effect that holds true for oscillating frequencies of VCOs manufactured using such inverters.
In contradistinction to the inverter case, when the differential amplifiers are used for the comparators 10 and 11, a charged potential across the capacitor 9 can accurately be compared with a reference potential. The result is that the oscillating frequencies of the VCOs manufactured are free from the manufacturing process of the transistors, resulting in substantial uniformity of the oscillating frequencies. However, a constant flow of the operating current in the differential circuit brings about great power dissipation of the whole VCO. For example, when the oscillating frequency is 2 MHz, the capacitance of the capacitor 9 is 5 pF and the charged voltage of the capacitor is 2.5 V; the charging current to the capacitor is approximately 50 .mu.A. The current consumed by each differential circuit is great; e.g. about 200 .mu.A. For this reason, in the case of the VCO using two differential circuits, most of the power dissipation by the VCO is that of the two differential amplifiers, and its value is large.
Furthermore, the differential circuit is constructed by analog technology. The analog circuit requires that a larger area of each transistor be used than with the digital circuit; for example, the inverter and the NAND gate. This fact indicates that when the VCOs are manufactured by integration technology, the semiconductor chip used is inevitably large in size.